1. Field of Invention
This invention is in the field of coating apparatus and methods therefor and, more particularly, is an apparatus for coating a substrate with a thermoplastic and method therefor.
2. Description of the Prior Art
A usual way of making a substrate impervious to harsh and corrosive environments is to coat it with a solvent based coating. A disadvantage of the solvent based coating is that the solvent includes a volatile organic compound that is a hazard to the health of people who handle it. Moreover, because of the inclusion of the organic compound, improper disposal of the solvent has a negative impact on the environment.
As an alternative to the solvent based coating, what is known as a flame spray process is used to coat the substrate with a polymer, such as thermoplastic. In the flame spray process, a flame is applied to a powdered thermoplastic at the mouth of an application gun that an operator points at the substrate. The flame fuses the thermoplastic. The fused thermoplastic is propelled by a stream of air from the mouth of the application gun to the substrate.
The temperature of the flame is on the order of 1800.degree. F. plus. The temperature causes a pyrolysis of a significant amount of the thermoplastic. The pyrolysis may result in an emission of toxic gasses that are a hazard to the health of the operator. Additionally, the temperature may result in a change of the molecular structure of the thermoplastic, known as cross-linking, that renders the plastic relatively ineffective.
The partial pyrolysis additionally creates a brightly colored flame, thereby preventing the operator from viewing the substrate while it is being coated. The operator must either frequently move the flame or repeat the flame spray process when the substrate is not completely coated. Therefore, the partial pyrolysis reduces the application rate of the coating.
It should be understood that the colored flame erupts in an explosive blast from the mouth of the application gun, thereby causing turbulence near the flame. The turbulence causes an overspray that increases the amount of thermoplastic that is used. Additionally, when the substrate is comprised of aluminum, for example, heat from the colored flame may cause an undesired change in the substrate; such as warping.
Heretofore, there has not been a simple, safe, economic and efficient way of coating the substrate with the thermoplastic.